According to periodic properties Theatomic radius is increases from top to bottom and decreases fromleft to right . Cs+is at IA group lastbutone in that group .So the radius of that ion is having more. And the remaining elements S andSe2+ comes under same group.They are having more radiusthan F . Among the S and Se2+ , S ia having small radius than Se2+.becausein groups size of the ion is increases. The priority Order is Cs+ > Se2+> S > F Fluorine got number 1 . and Cs+got number 4 Solution According to periodic properties Theatomic radius is increases from top to bottom and decreases fromleft to right . Cs+is at IA group lastbutone in that group .So the radius of that ion is having more. And the remaining elements S andSe2+ comes under same group.They are having more radiusthan F . Among the S and Se2+ , S ia having small radius than Se2+.becausein groups size of the ion is increases. The priority Order is Cs+ > Se2+> S > F Fluorine got number 1 . and Cs+got number 4.